1. Field of the Invention
This invention relates to improvement in a liquid crystal-aligning film of a polyether imide having superior liquid crystal-aligning properties and a high and controllable pretilt angle.
2. Description of the Related Art
As to liquid crystal display elements used for watches, electronic calculates, etc., a twisted nematic (hereinafter abbreviated to TN) mode exhibiting a structure having the aligning direction of nematic liquid crystal molecules twisted by 90.degree. between a pair of upper and lower electrode substrates, has currently been employed as a main current. However, as to this display mode, when it is applied to a high duty drive, the contrast is inferior due to cross-talk and the angle of view is narrow; hence it has been unsatisfactory for improving the display quality and the display surface. Recently, a liquid crystal display device making use of a super-twisted birefringence effect has been announced (T. J. Scheffer and J. Nethring, Appl. Phys. Lett., 45 (10), 1021 (1984)). Since then, a liquid crystal display element making use of a super-twisted nematic mode (hereinafter abbreviated to STN) having the aligning direction of nematic liquid crystal molecules twisted by 180.degree. to 300.degree. between the upper and lower electrode substrates has been developed; thus even in the case of a liquid crystal display element affording a large pictorial surface, those having a satisfactory display quality have come to be developed. In the case of an aligning film used for such elements, it is necessary to afford a definite angel (hereinafter abbreviated to pretilt angle) between the substrate planes and the liquid crystal molecules, in order to not only align the liquid crystal molecules, but also to improve response properties and ensure the bistability. Further it is preferred that the larger the twist angle, the larger the pretilt angle. Among these elements, in the case of those having a relatively small twist angle (180.degree. to 200.degree. twist), the interface treatment on the electrode substrates is sufficient with a cell provided with an aligning film having an interface of a currently generally employed pretilt angle (abbreviated to .theta.) of 5.degree. or less. However, in the case of those elements using a mode having a twist angle of 210.degree. to 300.degree. wherein they exhibit a better display quality, it is necessary to use an interface of a higher pretilt angle (5.degree.&lt;.theta..ltoreq.30.degree.); thus a liquid crystal display cell provided with an aligning film satisfying such a pretilt angle is necessary.
In the case of currently used aligning films of polyimides for a TN mode, display cells prepared on the commercial scale have a pretilt angle limited to 5.degree..
Japanese patent application laid-open No. Sho 61-240223 discloses a liquid crystal display element provided with a liquid crystal-aligning film using a polyimide resin having a repetitive unit expressed by the formula ##STR3##
Further, as a raw material for the unit, a concrete example of a diamine expressed by the formula ##STR4## is disclosed therein.
However, the polyimide aligning film obtained using this diamino compound has a drawback that no high pretilt angle is obtained as illustrated in a comparative example mentioned later. Further, polyimide aligning films having a high pretilt angle for a STN mode also exist, but they still have a problem in the aspect of the stability and reproducibility of the pretilt angle over the total area of the cell substrate having a broad display area.
European patent application laid-open No. 0389092 (filed by the same applicants as those of the present application) discloses a liquid crystal-aligning coating composed mainly of a polymer containing in its molecular chain, a substituted or non-substituted cyclohexylidene group expressed by the formula ##STR5## and also suggests as an example of the above formula, a formula wherein the above R.sub.1 represents the following formula and the above R.sub.2 to R.sub.6 each represent a hydrogen atom: ##STR6## wherein R'.sub.1 to R'.sub.6 each represent a hydrogen atom or an alkyl group of 1 to 15 carbon atoms.
However, polyimide-aligned films composed mainly of polymers containing the groups expressed by the above formulas have a considerably improved pretilt angle, which, however, is still insufficient. In order to obtain a high pretilt angle with certainty, a thin film formation by means of an oblique vacuum deposition with SiO.sub.2, etc. is the best method among those currently carried out.
However, the thin film formation by the vacuum deposition, when carried out by a commercial mass production, is more costly in terms of production apparatus; thus, it has been earnestly desired to obtain a superior liquid crystal alignment and a high pretilt angle by an interface treatment carried out by rubbing a thin film of an organic substance in the same manner as used in a conventional TN mode, and to realize the stability and reproducibility of the pretilt angle.